Conventionally, for the production of a circuit board comprising a base material and electroconductive wirings formed thereon, various methods have been used, such as a method of applying a photoresist onto a metal foil attached to a base material, exposing the photoresist to light through a desired circuit pattern, and forming a pattern by chemical etching. In this photoresist method, a metal foil is used to form electroconductive wirings, so that an electroconductive substrate with a small volume resistivity and high performance can be produced. On the other hand, this method has disadvantages such that it requires many steps and is thus a complicated method, and it requires photoresist materials.
Recently, a circuit pattern forming method has attracted attention, which forms a circuit pattern by directly printing a pattern on a base material, using a metal particle dispersion having metal particles dispersed therein and by a printing process such as screen printing or inkjet printing, and then sintering the metal particles. Compared to conventional photoresist methods, etc., the method of directly printing a pattern on a base material can increase productivity dramatically higher.
For metal particles, it is known that the melting point is dramatically decreased by reducing the size. This is because as the particle diameter decreases, the specific surface area increases and thus the surface energy increases. By using this effect, the sintering of metal particles can be promoted at lower temperature than ever before. Accordingly, it has been expected that it would be possible to form circuits on resin base materials with low heat resistance, which have been difficult to use, by printing. However, as the particle diameter decreases, it is more difficult to prepare a dispersion with excellent dispersibility and dispersion stability.
In Patent Literature 1, a method of forming circuit patterns by inkjet printing and using an electroconductive metal paste is disclosed. In Patent Literature 1, to prevent ultrafine metal particles from direct contact with each other, a method of covering the ultrafine metal particles with a compound is disclosed, the compound having a group containing nitrogen, oxygen and sulfur atoms that are able to coordinately bind to metal elements contained in the ultrafine metal particles. In Patent Literature 1, alkylamine, alkanethiol, alkanediol and the like are provided as the compound having a group containing nitrogen, oxygen and sulfur atoms, which is used to coat the metal surface. In Patent Literature 1, a relatively low molecular weight compound having 4 to 20 carbon atoms is provided as a concrete example, so that the alkylamine and the like can be detached from the metal, surface when the electroconductive metal paste is baked.
A metal-dispersed liquid is disclosed in Patent Literature 2, which contains metal particles having a mercaptocarboxylic acid and/or a salt thereof on the particle surface, a cationic surfactant and a non-aqueous solvent with low polarity. Patent Literature 2 describes that the mercaptocarboxylic acid or salt thereof on the metal particle surface is dissociated to be electrically negative in the solvent, and the mercaptocarboxylic acid ion is electrostatically bound to the cationic surfactant, thereby allowing the metal particles to maintain dispersion stability in the non-aqueous solvent with low polarity.
However, both the methods of Patent Literatures 1 and 2 disperse metal particles using a low molecular weight compound, and the dispersion stability of the metal particles is insufficient.
A metal microparticle dispersion is disclosed in Patent Literature 3, which contains a specific type of metal microparticles, a polymeric dispersant having a specific polyester skeleton, and a dispersion medium.
Patent Literature 3 describes that the specific polymeric dispersant have a high effect on the dispersibility of the metal microparticles and can be easily volatilized at a subsequent burning step. However, by the method of Patent Literature 3, the storage stability of the metal microparticle dispersion cannot be sufficient; therefore, the metal microparticles sometimes precipitate after a certain period of storage.
A metal-nanoparticle-containing composite is disclosed in Patent Literature 4, which comprises a metal compound, a specific polymer, a reducing agent and an aqueous solution, as a method for forming a metal film having a resistivity sufficiently low in terms of practicality through just drying at room temperature without any special heating-baking step or any step of removing a protective agent with a solvent. According to Patent Literature 4, in the aqueous solution, the sulfide-type sulfur functional group of the specific polymer and the phosphate residue of the same are likely to coordinate to the surfaces of metal nanoparticles; however, by removal of an aqueous solvent, they are dissociated from the surface to cause phase separation between the polymer and the metal nanoparticles, thereby allowing the fusion of the metal nanoparticles to proceed.
In this Patent Literature 4, an aqueous solvent is essential. In the case of using an aqueous solvent, poor coating properties on a base material are obtained, and blister formation is likely to occur; therefore, it is sometimes difficult to form a uniform film. Also in the case of using an aqueous solvent, poor drying properties are obtained; therefore, it takes time to dry a coating film and results in poor productivity. Because of the above reasons, in the production of electroconductive substrates, it is needed to form a film with an organic solvent in view of ease of handling.
Also in the method of Patent Literature 4, the specific polymer remains in the metal film, so that the resulting electroconductive substrate has insufficient electroconductivity and a problem with the adhesion between the base material and the metal film.